1. Field of the Invention
The present invention relates to systems and methods for logging wells to obtain information concerning the characteristics of underground structures. More particularly, the present invention pertains to nuclear logging techniques for determining the volume flow rates and flow directions of injected water moving behind the wellbore casing.
2 Description of Prior Art
In secondary and tertiary recovery of petroleum deposits, many of the recovery techniques employ the injection of water or chemical solutions into the earth formations comprising the reservoir from injection wells. Crucial information for proper planning of such a recovery operation includes the vertical conformity of the producing formations as well as their horizontal permeability and uniformity. Such information may be obtained by an evaluation of the direction and speed of formation fluid flow by a borehole in the field. By obtaining such information at a sufficient number of boreholes throughout the filed, a mapping of the total flow throughout a petroleum reservoir may be constructed to assist in the operational planning of injection of chemicals or water in the recovery process.
U.S. Pat. No. 4,051,368 assigned to the assignee of the present invention discloses techniques for analyzing gamma ray count data obtained from activated formation fluid to reveal the horizontal flow speed of the fluid.
In such recovery operations, it is also critical to know the flow dynamics of the injected fluid through the injection well borehole and into the formations. Typically, an injection well is cased and the casing perforated at the levels of the formations into which fluid is to be injected. As fluid is pumped down the injection well, varying proportions of the fluid pass through the perforations into the different formations. The patterns of fluid flow into the various formations, including the proportion of fluid passing into each formation are affected by the permeabilities of the formations themselves. However, the fluid flow pattern is also determined in part by the presence of vertical flow passages behind the injection well casing. Such vertical flow passages may be present in the underground structure itself. However, of particular concern are channels, or voids, which occur in the cement anchoring the casing to the wall of the borehole. Injection fluid passing through the casing perforations and exposed to such vertical passages is thus diverted upwardly and/or downwardly away from the formation intended to receive the fluid. Consequently, in order to plan for the injection of predetermined amounts of fluid within individual formations and to be able to monitor such fluid injection, a fluid injection profile of each injection well is necessary.
U.S. Pat. No. 4,032,781 discusses the occurrence of such vertical fluid communication in wells, particularly production wells. Such channels as well as naturally occuring passages may communicate fluid between a water sand structure, for example, and a producing formation, or even between two producing formations. Various methods of operation are described in the U.S. Pat. No. 4,032,781 for utilizing the technique of measuring vertical fluid flow by way of nuclear logging. Such methods of operation include not only the detection of fluid flow behind the wellbore casing but also include production profiling from spaced perforations within the casing. A logging sonde designed to measure vertical underground water flow behind casing lining a borehole is disclosed. A neutron accelertor is used to irradiate the flowing water with neutrons of sufficient energy to transform oxygen in the water into unstable nitrogen 16 particles. A pair of spaced gamma ray detectors monitors the radioactive decay of the N.sup.16 particles flowing with the water current. Linear velocity as well as volume flow rate values for the water current may be obtained by appropriately combining the measured radiation detection data.